It is important to know various numbering systems and their relationships in understanding thread size specifications; this Bulletin Post will serve as your quick reference guide. Figure 1 shows the primary terms useful in defining screw threads.
Form: The profile of the thread. Single thread: A thread having the thread form produced on only one helix of the cylinder. Thread sizes are communicated through various numbers and numbering systems which are derived by relating its unit length and weight.
Minor diameter tap drill diameter : The smallest diameter of a screw thread. A multiple thread permits a more rapid advance without a coarser larger thread form. Major diameter nominal diameterD: The largest diameter of a screw thread 6.
Crest: The edge or surface that joins the sides of a thread and is farthest from the cylinder or cone from which the thread projects. Communicating Thread Size with Ticket Numbers Introduction Thread selection for any specific application is based on many parameters, thread size is the primary consideration in achieving the functional and aesthetics requirements of the finished product.
External thread screw : A thread on the external surface of a cylinder. In this system, unit length is taken as fixed and its weight is measured.
Lead, L: The distance a threaded part moves axially, with respect to a fixed mating part, in one complete revolution. These elements may be used for aligning one part to another or may be used to transmit motion or force.